I'm pretty sure you aren't supposed to do this, but I'm wondering just the same. I have a beater saw that I bought for $5 at a flea market to learn how to sharpen my grandfather's saws and I realized that I don't have any that are crosscut. From what I've read the only difference is the bevel on the teeth (I think) that allows for the knife like cut so that could be done to a rip saw and make it a crosscut. I wouldn't do that to my family heirlooms but I might do that to a saw that someone had put lacquer on and mounted to a wall as a decoration (so sad). Anyway, looking for a little insight into this, thanks!
Rip saws have fewer teeth per inch than would a cross cut saw for cutting similar wood. They also have a different rake angle, the angle of the front of each tooth reference to the edge of the blade, as well as a fleem angle, the bevel angle between the face of the blade the the tooth.
Rather than going through all of that on this post, [link:www.vintagesaws.com/cgi-bin/frameset.cgi?left=main&right=/library/library.html|here is a web page] with all the info you should need.
I would recommend jointing the edge of the blade flat and recutting teeth of a more proper size for cross cutting. A method I like for spacing teeth is to make a paper gauge of the tooth pitch I want to file and tape it to the blade. [link:norsewoodsmith.com/node/76|Here] is a link on the [link:norsewoodsmith.com/|NorseWoodSmith] site that has links to pdf files of paper gauges for several different blade pitches. Scroll part way down the page and you'll find a text box with the links. The NorseWoodSmith site also has a ton of great info on saws and other hand tool stuff.